Method and apparatus for minimizing surface defects on itr cast strip

ABSTRACT

The formation of surface wrinkles, cracks and tears during Inside-The-Ring casting of strip is minimized by providing a slightly slower speed of the casting drum surface relative to that of the ring casting surface.

1111 3,811,491 [451 May21, 1974 United States Patent 1191 Gerding Hazelett.................,.l..........

[5 METHOD AND APPARATUS FOR 2,450,428 10/1948 164/277 MINIMIZING SURFACE DEFECTS ON [TR 3,627,025 12/1971 Tromel et a1. 164/87 X CAST STRIP FOREIGN PATENTS OR APPLICATIONS 1,177,758 1/1970 Great Britain...........,............ 164/87 [75] Inventor: Charles Christian Gerding,

Pittsburgh, Pa.

[73] Assignee: Jones & Laughlin Steel Corporation,

Pittsburgh, Pa. Primary ExaminerR. Spencer Annear [22] Filed Attorney, Agent, or Firm-T. A. Zalenski; G. K. White Mar. 1, 1973 Appl. No.: 337,022

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[56] References Cited UNITED STATES PATENTS surface relative to that of the ring casting-surface.

8 Claims, Drawing Figure 2,383,310 8/1945 Hazelett..........................'l64/277X METHOD AND APPARATUS FOR MINIMIZING SURFACE DEFECTS ON [TR CAST STRIP My invention generally is based upon the discovery that surface defects, such as wrinkles and cracks, which occur on the top surface of strip cast by the Inside-The- Ring process may be minimized by appropriate control of the respective speeds of the casting surfaces.

By the term lnside-The-Ring casting, it is meant the process in which a rotating drum and inside ring surface are cooperatively utilized as casting surface to form a solid two-sided strip casting from a molten pool of source material. In such process, as the ring rotates, a one-sided casting is continuously formed on the inside surface of the ring. The rotating drum extends into a predetermined portion of the molten pool which is contained on the inside surface of the ring and also produces a one-sided casting. Because the respective onesided castings are contiguous to each other, the two castings are continuously joined at the respective hot sides at the nip of the casting created between the drum and ring.

It has been discovered that the incidence of certain surface defects on the top surface of the resultant cast strip may be minimized by rotating the casting drum surface at a speed which is less than that of the inside surface of the casting ring by a slight, but controlled amount.

It is thus an object of my invention to provide a method and apparatus for minimizing the occurrence of surface defects such as wrinkles, cracks, and tears upon strip which has been cast by the lnside-The-Ring process.

It is an additional object of my invention to provide an apparatus that will provide for the attainment and control of slightly slower drum surface speed relative to that of the inside portion of the ring in an Inside-The- Ring continuous casting apparatus. These and other objectives and advantages of my invention will become more apparent from the following description thereof.

The FIGURE illustrates an embodiment of the apparatus which is suitable for accomplishing the method of my invention. As will become more apparent in a later portion of the specification, the apparatus provides for the use of an infinitely variable drive unit and differential gearing system in combination with an Inside-The- Ring casting device in order to effect the desired casting surface speed control.

The presence of surface defects on the top surface of two-sided lnside-The-Ring cast strip is a problem that has been associated with this particular technique of continuous casting. It may readily be seen that the solution to such problem is of significant economic benefit from the standpoint of increased material yield and quality level.

In order to provide a solution to the above problem, the relationship of the relative surface speeds of the drum and ring casting surface were investigated to determine what effect such relationship had upon the incidence of surface defects upon the top surface of continuously cast material. It has been determined that if the surface speed of the casting drum is controlled to be somewhat less than the surface speed of the ring, the formation of surface defects will be minimized.

It has also been determined that it is desirable for strip cast by the lnside-The-Ring process to follow and remain adjacent to the ring casting surface for some finite arc of contact upon exit from between the ring and drum. This is preferable to contact with the drum surface for several reasons. First of all, the strip should remain adjacentto one of the casting surfaces for the purpose of additional heat extraction. Secondly, as a greater strip thickness is cast upon the ring than that cast on the drum, the benefits of additional heat extraction can most advantageously be attained with contact with the cooling surface of the ring. Finally, as the ring forms a larger radius, a lesser overall bending strain will be imparted to the strip during final straightening if contact is made with the ring surface rather than the drum surface.

Various theories may be advanced to explain the exact mechanism which leads to the minimization or creation of surface defects upon the top surface of strip cast by the lnside-The-Ring technique. Such results are attributed, at least, in part to the geometry of the casting system. For example, it is self-evident that the top surface speed of a cast strip of finite thickness that remains adjacent to the ring will be slightly less than that of the bottom strip surface due to the fact that the top surface travels on a smaller radius than the bottom surface. In addition, it is believed that the top surface may be placed in tension during strip formation due to the partial unbending of the top strip (that portion initially cast onto the drum surface) as it is joined onto the bottom strip (that portion initially cast onto the ring surface) unless the top strip is cast in a manner that material is provided in such an amount or rate the undesired stretching or tension does not occur.

In order to illustrate the above point, it is first necessary to consider the situation where the drum and ring surfaces are driven at equal speeds. By driving both casting members at equal surface speeds, it necessarily follows that, prior to and at the point of welding the respective strips, the speed of the lower portion of the top strip and the speed of the upper portion of the bottom strip will not be equal. The speed of the lower surface of the top strip (that portion cast on the drum) will be greater than that of the drum due to radial differences. Conversely, the speed of the upper surface of the bottom strip (that portion cast on the ring) will be less than that of the ring due to radial differences. Hence, it clearly follows that a speed mismatch will occur at the welding or contact interface of the respective top and bottom strips; the upper strip bottom surface moving at a faster speed than the lower strip top surface. This condition will result in the creation of undesirable surface defects, such as wrinkles, upon the top surface of the resultant strip due to the fact that too much length of top strip is being created per unit length of bottom strip.

On the other hand, when the casting device is driven in a manner suggested by U.S. Pat. Nos. 2,383,310 and 2,450,428, an equally undesirable surface condition will be encountered. The above patents appear to teach a drive system in which only one of the casting members (the ring or drum) is driven. This would result in a situation in which drive energy is transmitted to the undriven member from the driven member at the location where the two cast strips are in contact, i.e., as they are welded together in the casting nip. Such arrangement will inherently result in unequal surface speeds of the respective casting members due to radial considerations. These geometrical considerations will I result in the drum having a surface speed which is too much less than that of the ring. As the welding interface provides the necessary driving contact, it also follows that the lower portion of the top strip and the upper portion of the bottom strip must travel at equal speeds. Such being the case, the strip welding will create an undesirable state of tension due to the fact that the top surface of the top strip contains less material than the bottom surface of the top strip. This is again due to geometrical considerations. The creation of a state of tension is undesirable because this leads to the formation of cracks and tears on the top surface of the resultant cast strip. This tendency would be greater with increasing strip thicknesses.

It may be seen that a technique which obviates the above conditions is desirable to effect the minimization of both types of surface imperfections. Thus, the invention minimizes the occurrence of tensile and compressive related surface defects by the expedient of controlling the relative speed of the casting surface members in a manner which avoids the creation of undesirable tensile or compressive forces on the top surface of the as formed strip. To accomplish this result it is necessary cause the drum to travel at a speed which is less than that of the inside surface of the casting ring by a slight, but controlled amount. This procedure is controlled so that the lower portion of the top strip moving at a rate which is greater than that of the upper portion of the bottom strip. However, such greater speed will not be sufficient to result in the formation of undesirable compressive forces and subsequent wrinkles. Instead, such strip speed difference is designed to compensate for the fact that additional material is needed for the top surface of the strip in order to match the amount of material which is created during the casting of the bottom strip so that the top of the top strip will not be stretched as it is continuously bent from the drum. This would result in the creation of a state of compression of a lesser amount than that resulting from equal drum and ring surface speeds, but of some finite amount so as to modify the effects from unbending the top strip. This degree of speed mismatch may conveniently be defined as rotating the first casting surface (drum) at a surface speed which is less than that of the second casting surface (inside of the ring) and will result in a greater strip speed for the bottom surface of the strip which is cast by the first casting surface than that portion of the resultant strip were it cast by the second casting surface. In other words, the relative surface speed of the first casting member would cause a strip cast upon the surface of the member to travel at a speed which is greater than if any corresponding portion of the strip were cast by the second casting member.

It is thus believed that the creation of a small, but finite, state of net compression is necessary to minimize the incidence of surface defects. However, it is not possible to place an absolute value upon the degree of casting surface mismatch that leads to the desired surface quality due to such variables such as equipment geometry and strip thickness. Therefore, the optimum casting surface speed mismatch for a given casting device and strip thickness is best determined through the result of empirical studies within the relationship discussed above. I

Thus, it may be seen that the invention is applicable in a method for the continuous casting of molten material where the molten material is caused to pass through a gap or opening between two casting surfaces which are rotating or moving in the same direction. It has been discovered, in such casting methods, that surface defects can be minimized by causing the casting surface which has the shorter radius of rotation to rotate at a surface speed which is less than that of the casting surface which has the longest radius of rotation. In the event that the strip thickness is desired to be changed during the casting process, the surface speed of at least the casting surface having the shorter radius can be varied in order to compensate for the new set of casting conditions. The method has particular utility for the continuous casting of ferrous and non-ferrous metals and alloy such as steel, copper, aluminum, etc.

The FIGURE depicts a preferredembodiment of an apparatus which is suitable for conducting the above described method. The apparatus generally comprises two contiguously positioned rotary casting members which are driven in the same direction and at different surface speeds in combination with means for delivery molten material into the gap created between the respective casting surfaces. The following is a detailed description of the FIGURE. Motor 18 drives drive pulley 17 which is connected to pulley 15 by belt 16.

Counter shaft 14, supported by pillow block bearingv assemblies 31 and 32 and driven by pulley 15, drives spur gear 13. Spur gear 13, in turn, is employed to drive spur gears 12 and 22. Spur gear 22 causes drive shaft 23, supported by pillow block bearing assemblies 24 and 25, to rotate and to thereby rotate drive rolls 26 and 27 and ring drive pinion 28. Drive rolls 26 and 27 rotate against the underside of ring 3 and both support the ring and assist in causing it to rotate. Ring 3 is also supported by idler roll 19 which is mounted on shaft 20 which in turn is supported by two pillow block bearing assemblies, 21 and one which is not shown. Ring drive pinion 28 meshes with ring gear 29 which is attached to side of ring 3 in order to drive the ring without consequent slippage. Thus, it may be seen that ring 3 will be driven to a constant degree which is a function of the output of motor 18. As mentioned previously, the same output is also transferred to spur gear 12 through spur gear 13. Spur gear 12 is connected to input shaft ll which is connected to infinitely variable drive unit and differential gearing system 10. Unit 10 comprises an infinitely variable mechanical speed changer unit in combination with differential gearing. One input of the differential is driven at a nominal constant speed while the other input is driven by the variable output of the speed changer. The output of the differential is the average of the two inputs, and the speed changer is geared down so that the differential adds the accurately adjustable trimming speed from the speed changer to a constant base speed which is the other input to the differential. If a common shaft drives both the constant input of the differential and the mechanical speed changer, then the output speed of unit 10 can be modulated up and down by modulating the speed of the common shaft. Adjusting means 33 may be employed to adjust the ratio of the speed changer and to thereby vary the output speed of unit 10. Adjustments of this nature are suitably accomplished in order to compensate for changes in strip thickness (strip thickness is a function of the gap created between the two casting surfaces which comprise drum 4 and ring 3.) It will be evident to those skilled in the art that the ratio of drum speed to ring speed can be made to vary automatically with strip thickness by an appropriate mechanism so that a constant amount of compression on the top surface may be realized even if the strip thickness is changed during operation. Infinitely variable drive unit and differential gearing system and adjusting means 33 are conventional and thus are not depicted in greater detail in the FIGURE. Suitable apparatus are commercially available and are described in greater detail in Book 3074 which is entitled P.I.V. Variable Speed Drives at pages 50-55 and is published by the FMC Corpora tion, Link-Belt Enclosed Drive Division. The controlled output from unit 10 is transmitted by output shaft 9 to drive sprocket 8 which drives chain 7 in order to turn drive plate sprocket 6. Drum drive shaft 5, connected to drive plate sprocket 6, is employed to support and cause casting drum 4 to rotate at the desired surface speed. Thus, drum 4 and the inside surface of casting ring 3 are caused to rotate in the same direction and at controlled speeds which are slightly different. Cast strip 1 is continuously cast between casting drum 4 and the inside surface of the casting ring 3. A pool of molten material 2 serves as the source of casting material and is fed by feeding device 34. As is known to those skilled in the art, aconvenient type of feeding device is a tundish. As various tundish feeding systems are conventional in the art, the drawing does not provide greater detail. Infinitely variable drive unit and differential gearing system 10 is mounted upon housing 30 which in turn is pivotally mounted upon the assembly which houses counter shaft 14. Such arrangement permits the height of casting drum 4 to be adjusted and also facilitates removal of casting drum 4 for servicing and maintenance of the casting apparatus.

I claim:

1. A method of continuously casting molten material,

comprising:

a. Forming a first cast strip on a first rotating casting surface and a second cast strip on a second rotating casting surface which rotates in the same direction as said first casting surface by contacting said casting surfaces with molten material, said first casting surface having a shorter radius of rotation than said second casting surface; 7

b. Forming a single casting by causing the respective cast strips to contact each other at a gap between said rotating casting surfaces so that the respective strips form a single casting whereby the bottom portion of said first strip is joined to the top portion of said second strip; and

c. Rotating said first casting surface at a surface speed which is less than that of said second casting surface and is controlled so that the bottom surface of said first cast strip travels at a speed which is greater than that of the top surface of said second cast strip when the respective strips are contacted between said rotatingcast surfaces whereby the occurrence of surface defects upon said single casting is minimized. 2. A method of continuously casting molten material as recited in claim 1, wherein the speed of at least said first casting surface is varied with changes in solid casting thickness.

3. A method of continuously casting molten material as recited in claim 1, wherein said material comprises a metal.

4. A method of continuously casting molten material as recited in claim 3, wherein said material comprises a ferrous metal.

5. Apparatusfor the continuous casting of molten material, comprising: I a v a. A first rotary casting member having a surface adapted to cast molten material;

b. A second rotary casting member having a surface adapted to cast molten material, a larger radius of rotation than that of said first rotary casting member, and positioned contiguous to said-first rotary casting member so as to form a gapbetween said casting surfaces;'and

c. drive means connected to said rotary casting members for causing rotation of said members in the same direction and for causing said first rotary casting member to rotate at a surface speed which is less than that of said second rotary casting member and which would also cause a strip castupon the surface of said first casting member to travel at a speed greater than if any corresponding portion of the strip were cast by said second casting member.

6. Apparatus for the continuous casting of molten material as recited in claim 5, wherein:

said drive means comprise an infinitely variable drive unit and differential gearing system.

7. Apparatus for the continuous casting of molten material as recited in claim 5, wherein:

said drive means further comprise-adjusting means for varying the relative surface speeds of said rotary casting members so as to compensate for differences in said gap between said casting surfaces.

8. Apparatus for the continuous casting of molten material as recited in claim 5 in combination with feeding means for providing molten material for casting. 

1. A method of continuously casting molten material, comprising: a. Forming a first cast strip on a first rotating casting surface and a second cast strip on a second rotating casting surface which rotates in the same direction as said first casting surface by contacting said casting surfaces with molten material, said first casting surface having a shorter radius of rotation than said second casting surface; b. Forming a single casting by causing the respective cast strips to contact each other at a gap between said rotating casting surfaces so that the respective strips form a single casting whereby the bottom portion of said first strip is joined to the top portion of said second strip; and c. Rotating said first casting surface at a surface speed which is less than that of said second casting surface and is controlled so that the bottom surface of said first cast strip travels at a speed which is greater than that of the top surface of said second cast strip when the respective strips are contacted between said rotating cast surfaces whereby the occurrence of surface defects upon said single casting is minimized.
 2. A method of continuously casting molten material as recited in claim 1, wherein the speed of at least said first casting surface is varied with changes in solid casting thickness.
 3. A method of continuously casting molten material as recited in claim 1, wherein said material comprises a metal.
 4. A method of continuously casting molten material as recited in claim 3, wherein said material comprises a ferrous metal.
 5. Apparatus for the continuous casting of molten material, comprising: a. A first rotary casting member having a surface adapted to cast molten material; b. A second rotary casting member having a surface adapted to cast molten material, a larger radius of rotation than that of said first rotary casting member, and positioned contiguous to said first rotary casting member so as to form a gap between said casting surfaces; and c. drive means connected to said rotary casting members for causing rotation of said members in the same direction and for causing said first rotary casting member to rotate at a surface speed which is less than that of said second rotary casting member and which would also cause a strip cast upon the surface of said first casting member to travel at a speed greater than if any corresponding portion of the strip were cast by said second casting member.
 6. Apparatus for the continuous casting of molten material as recited in claim 5, wherein: said drive means comprise an infinitely variable drive unit and differential gearing system.
 7. Apparatus for the continuous casting of molten material as recited in claim 5, wherein: said drive means further comprise adjusting means for varying the relative surface speeds of said rotary casting members so as to compensate for differences in said gap between said casting surfaces.
 8. Apparatus for the continuous casting of molten material as recited in claim 5 in combination with feeding means for providing molten material for casting. 